The present invention is concerned with a process for treating with a plasma objects made of electrically conductive materials. The invention is concerned, in particular, with the treatment by an atmospheric plasma of objects having the shape of foils or wires, for example after their rolling or drawing.                The treatment of a material can include one or several of the following operations:        Cleaning one or of two faces of a foil or of a wire, to remove organic residues and technological oils and greases.        Deodorising.        Disinfecting, sterilising.        Activating the surface for improving the adhesive properties or the wettability.        Modifying the surface layer by fusing micro-cracks, polishing or forming a superficial alloy.        Stripping.        Annealing, elimination of internal stresses.        Depositing films.        
It is known how to use processes and devices for the cleaning of surfaces of rolled foils or of wires to remove therefrom the remains of the greases used in their manufacture. The treatment devices, the most frequently used, are diffusion furnaces, gas burners, chemical baths, steam jets, devices producing electrical discharges of the barrier type or silent discharges and plasma jet devices.
The drawbacks of diffusion furnaces, for the cleaning of metal foils, whether they use air or oxygen, is that they have high electrical power requirements and their action is very slow. Days, if not weeks, are needed for a completely cleaning of the foils that are normally loaded into the furnace in the form of rolls. The process consists in heating the roll, in making air or oxygen penetrate between the layers of the foil in order to oxidise the organic residue and to remove, mainly by diffusion, the residual gas from between the layers of foil tightly pressed one upon the other in the roll. Accordingly, the productivity and the efficiency of such furnaces is low. The diffusion furnaces are only used for the cleaning and the annealing of materials.
Linear flame burners are little used, owing to their low efficiency and to the limited variation of the composition of the flame containing carbonic products. Flame burners are generally used only for the cleaning and for the annealing of materials.
Chemical baths use considerable amounts of chemical products, and these products need to be recycled. The process has its limitations, due to the high cost of recycling and the need to comply with increasingly strict ecological standards.
Steam jets are used for the cleaning of wires. This cleaning, given its low efficacy, is used practically only for a preliminary coarse cleaning operation, which is generally followed by a chemical cleaning.
The use of discharges of the barrier type or of the silent type, which offers the advantage of a uniform treatment of the material, is not suitable for industrial purposes owing to the limited productivity of these processes.
The treatment by atmospheric plasma jets is relatively effective, since a highly activated gas carries out this treatment and the oxidation is the result of a plasmo-chemical reaction. Furthermore, the productivity is high, owing to the high power density achievable. The plasma jet treatment devices are generally simple, inexpensive and adaptable. Also, one can carry out a cleaning of a surface, an annealing, a stripping or a deposition of a film, the composition of the plasma gases being adapted to the treatment to be carried out.
A disadvantage of the processes using plasma jets is that it is difficult to achieve a uniform treatment over the whole surface of the foil or of the wire.
To remedy this drawback, some authors have tried to design devices using linear plasmatron assemblies, to create plasma curtains.
For example, in the publication WO 97/18 693, a plasma generator is described, which generates a plasma having the shape of a curtain, by superposing several jets produced from several plasmatrons having two nozzles. The drawback of this process is that the desirably uniform distribution of the parameters, such as the temperature, cannot be achieved in most industrial applications of interest. Actually, the flow of gas through the plasmatrons and the speed distribution do not make it possible to obtain, in all cases, the desired uniform distribution of the parameters. Even when the temperature is uniform over the whole length of the curtain, this does not necessarily imply that the speed and/or the composition of the plasma will be uniform, and this results in a lack of uniformity of the treatment.
Furthermore, since the use of this type of linear plasma generator for the treatment of metal foils necessitates a rapid passage of the foil through the plasma curtain, the moving foils drag along an important flow of gas or of air. This turbulent flow of gas interferes with the flow of the plasma, with the result that the plasma is cooled, looses its energy that is in the form of potential energy of its particles and becomes turbulent. The efficiency of the treatment is strongly decreased by the cooling. The turbulence can cause deformations of the foil being treated, such as crumpling, mainly in the case of thin foils.